In numbers, gamma delta T cells represent a significant component of the immune system in widely diverged vertebrates, including humans. Their function in the body remains, however, unknown. We proposed in 1988 that they were in large part responsible for the protection of surface epithelia against primary infection, reasoning that they may recognize 'stress' antigens. While much data supports this assessment, there is still very little data concerning what happens when animal epithelia are systematically infected. This study focuses on the response of gamma delta T cells to protozoans that infect animals across the vertebrate phylum. The experiments will be supported by others that will identify the proteins on other cells that are recognized by gamma delta T cells. Together, these experiments will help us understand how this newly-discovered set of lymphocytes helps protect animals from disease.